Damping-off occurring in South Dakota soybeans

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With soybean planting nearing completion and soybean plants emerged or starting to emerge, now is the time to check your fields for seedling diseases associated with damping-off and also to assess plant stand establishment, says Connie Strunk, Plant Pathology Field Specialist, South Dakota State University.

"We are starting to see some damping-off occurring out in the field. Damping-off can reduce plant stand per unit area. If small skips within a row are noticed, soybeans have the potential to compensate for a few lost plants. However if larger skips or the entire row of soybean has missing plants (Fig. 2), this will impact yield," Strunk said.

Although seed treatments offer some protection, it's for a limited time.

"Protection against seed and seedling diseases is for a limited time as the fungicides are effective for about two to three weeks after planting, depending on soil temperature and soil water content," she said.

Damping-off can occur two ways: before emergence, where the seedlings fail to emerge because of a fungal attack; or post-emergence, where the seedlings are killed by fungal attack after emergence.

"Since the soils were wet and cool - soil temperatures less than 60o Fwhen planting took place we expect to see soybeans attacked by Pythium, especially in wet spot or low-laying areas of the field," Strunk said. "In addition to the cool and wet conditions, fields that have heavy or compacted soils or high amounts of residue are deemed a higher risk for Pythium infection."

She explains that Pythium is a pathogen that may attack the soybean seeds before or after germination until the second trifoliate (V2) growth stage. After this growth stage, soybean plants are resistant to Pythium infection. Soybean plants may be killed by damping off before or after emergence has taken place. The hypocotyl on seedlings infected by Pythium is typically narrow and is often pinched off.

Other pathogens which may attack soybeans are Phytophthora and Rhizoctonia (which are often a problem in warmer soils, 70- 80 F). Phytophtora typically has a brown discoloration extending from the soil line up the stem of the soybean plant. Rhizoctonia is often seen in late planted soybean fields. With Rhizoctonia infection you will often see a reddish-brown colored lesion on the hypocotyl and lower stem of the plant which is not observed above the soil line.

Even though most soybean seed has been treated with a fungicide seed treatment, no seed treatment product is effective against all seedling diseases and under all conditions. Strunk encourages growers to look at what protection their seed treatment offers and the performance it delivers.  

"If your soybean field has been affected by seedling blight or damping-off, it is helpful to identify what seedling disease caused the infection," she said. "Proper diagnosis will help with your fungicide seed treatment selection for replanting purposes and for planning next year's crops."

 

When planning their 2014 seed selection, Strunk encourages growers to rotate soybean cultivars with different Phytophora resistance genes if Phytophthora root rot occurs in their fields.

"The Phytophthora pathogen has many races, and depending on what races are occurring in one's field, this will determine the effectiveness of the resistance genes in the cultivar being planted," Strunk said.


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